Copper plates have conventionally been bonded to an alumina substrate by two methods. In the first method, a copper paste containing a binder is coated on an alumina substrate and subsequently dried and fired so that the binder in the paste is caused to react not only with the glass forming material in the alumina substrate but also with the copper plate itself. In the second method, a reactive metal (brazing filler material) sandwiched between an alumina substrate and a copper plate is heated at a temperature higher than the melting point of the reactive metal so that the copper plate is bonded to the alumina substrate through diffusion that occurs between the substrate and the reactive metal and between the copper plate and the reactive metal.
With the recent trend for semiconductor devices to be directed toward larger power consumption, higher scales of integration and use of modules, there has been a growing need to increase their efficiency with respect to heat dissipation from the ceramic substrate, to simplify the procedures for packaging semiconductor chips and to improve the reliability of the devices. In response to this need, various methods have been proposed by which a copper circuit board can be directly bonded to an alumina substrate without using the conventional copper paste or brazing filler material.
British Patent No. 761,045 to Lodge Plugs Limited describes a method for bonding copper directly to a ceramic substrate. In this method, a copper plate that has been strongly oxidized is positioned on the alumina substrate and the combination is heated to a temperature that is higher than 1,083.degree. C. (the melting point of copper) but which is lower than the melting point of cuprous oxide (ca. 1,200.degree. C.). During the heat cycle, the majority of the cupric oxide present is converted to the cuprous form and the copper plate becomes molten but the cuprous oxide remains unmelted to form a layer at the interface between the alumina substrate and the copper plate.
Japanese Patent Publication No. 4154/1985 filed by General Electric Company claiming priority from U.S. Ser. Nos. 600,300 and 696,899 (inventors: D. A. Cusano et al.) describes an invention entitled "A Method for Bonding Copper Members to a Ceramic Substrate". According to this method, a copper oxide 25 layer having a thickness of 200-5,000 A is formed on the surface of either a copper member or a ceramic substrate before the copper member is positioned on the substrate and the combination is heated in an inert atmosphere to a temperature between 1,065.degree. C. (the eutectic temperature of the copper-copper oxide eutectic) and 1,083.degree. C. to produce a hypoeutectic melt of copper and copper oxide at the interface between the copper plate and the substrate, and is subsequently cooled.
The two prior art methods described above differ from each other in that the combination of copper plate and alumina substrate is heated to a temperature higher than the melting point of copper (1,083.degree. C.) in the first method whereas the combination is heated to a temperature below that point. However, they are the same in that the copper plate is directly bonded to the alumina substrate without employing an interposing material that wets both copper and alumina. In the latter method, a liquidus eutectic of copper and copper oxide is used as a bonding agent.
However, the method described in Japanese Patent Publication No. 4154/1985 has the disadvantage that the wetting effect of the liquidus eutectic formed at the interface of the bond between the copper plate and the ceramic substrate can cause displacement of the former over the latter. The present inventors confirmed by experiment that the problem of such displacement was pronounced when a small copper plate was to be bonded to the alumina substrate.
The formation of the liquidus eutectic is also disadvantageous in that it impairs the inherent flatness of the copper plate and should be avoided as much as possible if the bonded assembly of copper and alumina is to be used as a substrate for mounting electronic components because surface roughness is one of the important parameters determining device reliability.
When bonding is effected using a eutectic melt, there is a high likelihood that bulging will occur during solidification of the melt on account of oxygen. Bulging present in the bonded assembly of copper plate and alumina substrate retards heat dissipation and is detrimental to the efficiency of the subsequent assembly operations.
Therefore, although the method described in Japanese Patent Publication No. 4154/1985 is capable of providing a satisfactory assembly of a copper member directly bonded to an alumina substrate, the performance of the assembly as a board for installing electronic components and its production rate are still insufficient to meet current requirements which are becoming steadily more rigorous as a result of the recent rapid advances in semiconductor technology. Furthermore, there still remains room for improvement in terms of such aspects as high-volume production and cost reduction.